Integrated circuits, in particular RFID circuits are subject to strict rules for testing. This is particularly the case when such integrated circuits are used in automotive applications, where a high level of safety and security is required. For example, it is often required to subject these circuits to stress testing, wherein a limited number of the device is operated at high temperatures for many hours (e.g., more than several thousand hours) in order to provide a reliable forecast of the aging behavior of the devices of a production cycle over their whole lifetime. However, the peak temperature the devices can withstand is limited. Therefore, the amount of time the devices are to be stressed can not be reduced below a minimum, which provides that a large number of devices are to be stressed simultaneously in order to receive reliable statistic results. Stress testing so many devices at once under harsh conditions is difficult and it is challenging to design test equipment that can cope with these conditions.
Furthermore, some devices under test (DUTs) are envisaged for bidirectional operation or communication; i.e., they can receive and they can transmit signals over the same lines or connections. The stress test should, therefore, be able to handle both directions.
For example, passive RFID transponders are charged during a first charging phase where an RF signal is transmitted from the Read/Write unit to the transponder (downlink). The signal is received through a resonant circuit, which is typically implemented as an LC resonant circuit. The antenna constitutes the inductor (i.e. the L) which is coupled with a capacitor (i.e. the C). One of the components or both, i.e. L and C may be implemented as discrete components, i.e. they are not integrated on the same integrated circuit with other components of the RFID chip. After having received the initial RF signal during the charging phase for a sufficient amount of time, the RFID transponder can start operating from its internal power supply (e.g. a capacitor), which was charged by energy transmitted as an RF signal during the charging phase. The RF signal was rectified and stored as electrical charge on a capacitor. Active RFID transponders may use a battery or the like as a power source and a charging phase may not be required. However, active and passive transponders may use bidirectional signal transfer via the external components, i.e., the LC resonant circuit.
For bidirectional data transmission, an RFID transponder chip is adapted to receive data from a Read/Write unit (downlink) and to transmit data to the Read/Write unit (uplink). An internal clock signal may be used in the RFID transponder chip for performing data processing, control tasks and data transmission, in particular, in during uplink. Therefore, the RFID transponder chips may have an internal oscillation maintenance stage, which establishes and maintains an internal oscillation or clock signal when no external RF signal from a Read/Write unit is present. The oscillation maintenance circuit may use a plucking mechanism in order to maintain the oscillation. The plucking mechanism may periodically couple the LC resonant circuit to ground or a supply voltage in order to supply energy to the LC resonant circuit so as to compensate losses and maintain the oscillation of the LC resonant circuit.
In one known method of stress testing integrated circuits, all the integrated circuits (for example 100 or more) are directly coupled parallel to one signal generator via a transformer. This allows signals from the signal generator to be supplied to the integrated circuits under test. However, in the known method the devices can not transmit signals. The devices under test can only operate in one of the two transmission directions for an integrated circuit (e.g., for an RFID transponder, only in the downlink direction and not in the uplink direction).
Another known method of stress testing is to supply each device by its own transformer. However, for example, for passive RFID transponders with an oscillation maintenance circuit, which is activated after a charging period of the device, it may be necessary to disconnect this signal generator during self-oscillation phases, when the oscillation maintenance circuit will be enabled. Otherwise, the outputs of the devices under test may be shortened by any low impedance of the signal generator output (for example, 50 Ohms). In order to achieve this disconnection, fast analog switches having a modulation frequency (for example, 1 to 2 kHz) for high voltages (around 45 V, for example) are necessary. Additionally, an exact synchronization of the signal generator and the control signal for the analog switches is necessary to prevent over voltage conditions. Furthermore, the analog switches can be damaged due to the harsh conditions of stress testing at high temperatures and thus the devices under test fail the qualification tests, although the failure is due to the switches.